1. Field of the Invention
The present invention relates to a bias voltage stabilizing method for an electricity-optical modulator based on an off-level sampling, and in particular to an improved bias voltage stabilizing method for an electricity-optical modulator based on an OFF-level sampling which is capable of stabilizing an output characteristic of a Mach-Zehnder (MZ) interference type optical modulator formed of a LiNbO.sub.3 or electrical-optical polymer.
2. Description of the Conventional Art
Generally, the operation level of an electricity-optical modulator is different in accordance with an application system. For this, a DC voltage may be applied to a signal electrode or an additional bias voltage electrode may be provided for controlling an operation level of a modulator. However, a conventional electricity-optical modulator has a DC drift phenomenon with respect to the DC power. In addition, the operation level may be changed due to a surrounding temperature and moisture, stress, etc. This operation level variation may result in a distortion of a high speed modulated optical signal and a degradation of an extinction ratio.
FIGS. 1A and 1B illustrate an output type based on an initial operation level of an optical modulator when the identical digital signals are inputted. As shown in FIG. 1A, a digital signal is inputted as indicated by "1" and is converted into an optical signal by the optical modulator having a modulator output characteristic as indicated by "2", and is outputted as indicated by "3". At this time, the initial bias voltage level "a" which is the initial level of an output characteristic curve of a modulator is coincided with the bias voltage level of an electrical signal. In addition, in this case, it means that the electrical signal is well converted into an optical signal.
However, as shown in FIG. 1B, the operation level which is the minimum level of a modulator output characteristic curve is shifted from the initial bias voltage level. Therefore, when the same is shifted to the level position of the electrical signal "1", the optical signal as indicated by "3" is distorted, so that it is impossible to identify a high bit or low bit. When the same is shifted away from the initial bias voltage level, the output signal level of the converted optical signal may be changed.
Therefore, it is needed to control the operation level. For this, an additional bias voltage is installed at the optical modulator for thereby controlling the operation level. However, an initial optical modulator operation level determined based on a DC bias voltage applied to an optical wave guide layer may be changed due to a photo refractive effect of a medium, a DC drifting phenomenon based on an accumulation of a space charge, a surrounding temperature variation, etc.
In order to continuously maintain an initial state of an operation level, the bias voltage is changed in accordance with an output state of an optical modulator, an output light stabilizing method is needed.
One of the most used methods is a stabilizing method using a dithering signal as shown in FIG. 2.
As shown in FIG. 2, a second harmonic signal is extracted from a dithering signal 1 using a band pass filter 2, a frequency doubler 3, and a phase controller 4. An optical signal from the optical modulator 5 is detected by a photo detector 6 and passes through a low noise amplifier 7. The optical signal outputted from the low noise amplifier 7 is controlled by a lock-in amplifier 8 in accordance with a second harmonic signal of the dithering signal outputted from the phase controller 4. The output signal from the lock-in amplifier 8 is inputted into an adder 12 through a low band pass filter 9 and an integrator 10. A fundamental wave signal of the dithering signal 1 passes through the band pass filter 2 and a DC bias voltage signal inputted from the DC bias voltage unit 11 are added with the signal from the integrator 10 by the adder 12 for thereby controlling the optical modulator 5.
A fundamental wave of a dithering signal modulated by applying a 1.about.100 kHz small electrical envelop dithering signal to the input light or a second harmonic signal is extracted for thereby obtaining an error signal, and then the bias voltage is controlled based on a feed-back control using the thusly obtained signal.
If the second harmonic signal of the dithering signal is extracted, and an error signal is used, the error signal value becomes 0(zero) at the level in which the linearity is best, and at this level, the code of the level of the error signal is changed. Therefore, it is possible to stabilize the position of the operation level of the optical modulator 5 at the level in which the linearity is best by controlling the DC bias voltage 11, namely, a reference voltage inputted into the adder 12 in accordance with the level of the output signal of the lock-in amplifier 8.
However, when forming the feed-back circuit using a fundamental wave of a dithering signal of the optical modulator or a second harmonic wave as an error signal, the values of the error signal to be checked in accordance with the traffic character is changed. Therefore, the lock-in detection method based on the known dithering signal is not proper for the optical gate application of the optical modulator.
FIG. 3 illustrates a circuit for controlling the bias voltage for providing the identical traffic characteristic compared to a valid DC component of an electrical input signal and an optical output signal based on the conventional method.
An input signal is inputted into an optical modulation driver 24 as a modulation signal 23, and an optical modulator 22 receives a source light of a laser diode 21 and changes a modulation signal 23 into an optical signal in accordance with a driving of an optical modulation driver 24. At this time, the signal passed through the low pass filter (LPF) 25 and the reference voltage 27 are differentially amplified by a first differential amplifier 26 with respect to the modulation signal 23, and an optical signal outputted from the optical modulator 22 is detected by the photo detector (PD) 29 and passes through the low pass filter 30 and then an optical signal is detected based on a pre-amplifier 31, and an output signal from the pre-amplifier 31 and an output signal from the first differential amplifier 26 are differentially amplified by a second differential amplifier 32 and then the thusly amplified signal is inputted into a PI (Proportional Integrated) control circuit 33, so that the PI control circuit 33 controls the output characteristic of the optical modulator 22.
The modulation signal 23 which is an electrical signal inputted into the optical modulator 22 is filtered using a low band pass filter 25 for thereby obtaining a reference voltage proportional to the amount of the traffic of the signal. In addition, an optical output from the optical modulator 22 is measured and then is filtered using the low pass filter(LPF) 30 for thereby obtaining a DC voltage proportional to the traffic signal of the output light. If the output signal from the optical modulator 22 is modulated identically to the electrical signal, the DC voltage (namely, the output voltage of the pre-amplifier 31) of the output of the optical modulator 22 and the value of the electrical reference voltage (namely, the output voltage of the first differential amplifier 26) should be identical. Therefore, the voltage difference between the reference voltage of the electrical signal and the optical output is obtained by the second differential amplifier 32, so that the optical modulator 22 is modulated based on the DC control circuit 33 in accordance with the error signal. Therefore, it is possible to obtain an optical output modulated identically to the electrical signal inputted.
The above-described method may be adapted to the burst traffic characteristic, but uses an error signal proportional to the absolute value of the DC voltage. Therefore, the circuit may be affected by an unstable operation of the electronic circuit devices due to a temperature, humidity and stresses etc. As a result, the above-described method is not adapted to the burst optical gate application of the optical modulator.
In the modulation of an analog signal and a digital signal transmission, since a signal distortion problem may occur due to the periodic output characteristic of the optical modulator, an operation level at which the output characteristic curve of the optical modulator is best is fixed. In this case, a small dithering signal may be carried on the bias voltage electrode or a signal is modulated into a small dithering signal of a low frequency. Therefore, the level in which the second harmonic wave of the optical signal modulated by the dithering signal is the level in which the linearity is best. Namely, the level corresponds to the level in which the phase difference is .PI./2. Therefore, the above-described method is generally used.
However, in the case that the electricity-optical modulator is used as an optical gate, it is important to maintain a maximum extinction state of the ON/OFF state of the output light. Namely, the OFF state should be in a minimum pass-through state of the optical modulator.
Therefore, for using an application of the optical gate, in the case that an electrical input signal is OFF, the bias voltage of the modulator is controlled to be at the minimum level of the output of the modulator. For this, if the dithering signal method is used, since the fundamental frequency component is the lowest at the minimum level of the pass-through curve, it is possible to control the bias voltage based on the method of minimizing the fundamental dithering frequency component of the output light.
However, in this case, it is assumed that the traffic characteristic of the input signal is a uniformly scrambled data or is constantly defined. If the input signal has a burst characteristic like the input signal is continuously ON or OFF, the size of the error signal for the output stabilization may differ in accordance with the traffic characteristic, so that it is impossible to implement an adaptive control of the optical modulator.
The problems that in the conventional dithering method, it is difficult to implement an adaptive control in accordance with the traffic characteristic are caused for the reason that the error signal is extracted in the ON and OFF states of the output light.